My first question is what mechanical broadhead are you using? The second one is how do you know they didn't open? I ask because when I used Spitfires and pulled them out, sometimes they would close back up on me. The only way I knew that they opened was because some of the foam would get under the blades. Check the blades and the channel they fit into for foam to make sure that they are opening on contact. This applies to any mechanical broadhead but the ones that use rubber o-rings are easier to verify that they opened. Usually the o-ring gets pushed back onto the arrow shaft.

Steve